The establishment of a vertical reference above an obscure datum by a surveyor or for a construction worker to transfer a datum vertically from one level to another are everyday requirements in the surveying and construction fields. One of the most common items to accomplish these functions is the plumb bob. A plumb bob comprises a weight affixed to a flexible member such as a string so that the string may be held vertically above a reference point or datum. The string, therefore, becomes an extension of the reference point or datum that is located underneath the weighted end of the string. In order to increase the accuracy of such a system, the weight has for many years been made in a conical shape. Representative of such a device is the plumb bob described in U.S. Pat. No. 2,381,698 issued on Aug. 7, 1949 to G. Sireci. Plumb bobs such as the Sireci plumb bob have traditionally been made of metal, usually brass. In recent years, some plumb bobs have reached the field made of some other metal, with a chromium outer surface.
When a surveyor utilizes a plumb bob, the rod man or the chain man suspends the plumb bob by the string so that the pointed end of the plumb bob is directly over the datum. The instrument man, when using a transit, will ideally sight in at the lower end of the plumb bob if it is visible; however, sighting on the string or an associated target located on the center line of the string may suffice if either the datum or plumb bob is not visible. The purpose of sighting in on the pointed end of the plumb bob itself is to ensure greater accuracy. With traditional brass plumb bobs, the surveyor may have difficulty seeing the plumb bob itself should there be vegetation or other material of a similar color in the vicinity of the datum. The use of a chromium plated plumb bob could, in part, overcome this problem; however, reflections from the chromium-plated plumb bob, when received in the magnifying telescope of the transit, could cause serious injury to the eye of the surveyor. Accordingly, it is appropriate to make a plumb bob of a high visibility non-reflective material so that it is easy to see at a distance. It should also be noted that the high visibility material facilitates locating a plumb bob if it is dropped in high grass or in some other obscure place.
In the past, plumb bobs have usually included a steel point since the plumb bob is easily dropped and damage to a softer metal such as brass easily occurs. Generally, the steel point is replacable, as is disclosed in the Sireci patent. Most, if not all, plumb bobs utilizing a replacable point such as Sireci require a specific type of point not readily available to the surveyor in the field. Thus, should the instrument man, rod man, or chain man, drop and damage the point to the plumb bob, the surveying work must come to a halt until a replacement point or another plumb bob is obtained. Therefore, in addition to the high visibility of a plumb bob, it is appropriate to provide a plumb bob that includes provision for an easily-obtainable replacable point, should the hardened steel point become damaged or lost.
Plumb bobs such as that disclosed by Sireci ordinarily carry at their upper end some means for affixing the string or flexible member. While Sireci teaches a rotating cap such that the knotted end of the string may be placed through an opening and the cap rotated so that the string is positioned at the center line of the plumb bob, the more common plumb bob utilizes a knurled screwed-on cap having a central bore. Thus, to affix the string to the common plumb bob, the cap is unscrewed, the string threaded through the bore and knotted, and the cap replaced on the plumb bob. While this system has worked with efficiency in the past, it is subject to the loss of the cap upon removal of the cap from the plumb bob. If the cap is lost, for example, in high grass, the surveying work must again come to a halt until the entire plumb bob can be replaced. It should be noted that it is not uncommon for the string to break after prolonged use of the plumb bob.
As previously noted, the supporting string of a conventional plumb bob is threaded through a bore in a removable cap. This cap of the plumb bob is made so that the bore and the outer surface of the plumb bob meet at an angle. This angular junction either abrades the supporting string, causing it to break, or in the extreme cuts the string when tension is placed thereupon. Further, in order to facilitate removal of the cap, the edge thereof is knurled. This knurled edge will also tend to abrade the string, causing early failure. When the surveyor uses a spring-activated surveyor's reel in conjunction with the plumb bob, the reel will retract the string up to the plumb bob. Then the surveyor will place the plumb bob in its sheath so that the reel will hang relatively freely from the top or cap of the plumb bob. As the surveyor moves about the angular edge and the knurl will continuously work on the string until the string finally wears through so that the reel drops off the plumb bob and possibly is lost. Accordingly, this invention has as one of its objects a plumb bob that has a smoothly curved top portion to virtually eliminate abrasion to the supporting string.
Finally, it should be noted that a metallic plumb bob provides a given weight for a given volume, thereby stabilizing the plumb line to the degree desired by the user. (Of course, the heavier the plumb bob is, the more stable it will be in wind conditions.) Since metallic material is relatively dense, it is particularly appropriate for such material to be used in the manufacture of plumb bobs.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide alternatives to this prior art. In one aspect of this invention, this is accomplished by providing a plumb bob, having a conical portion and including a knobbed extension. The knobbed extension defines an axial bore and a smoothly curved top portion connecting the outer portion of the extension with the bore.
This plumb bob, which may include a shell made of a high-impact plastic having a high-visibility color, may also permit easy replacement of a pointed member affixed at its smaller end in the field by the use of conventional concrete nails.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. It is to be especially understood, however, that the drawings are not intended as a definition of the invention, but are for the purpose of illustration only.